1. Field of the Invention
This invention relates to a dry etching method for manufacturing semiconductor devices.
2. Description of the Related Art
Etching has been hitherto known as a technique used in the fabrication of semiconductor devices, and several etchings techniques are developed. Among these techniques, reactive ion etching (hereinafter known as RIE) has been widely employed in the field of anisotropic etch which utilizes gases in the plasma state including fluorine compounds or chlorine compounds. Such a technique is described in "Applied Physics" in 1984, vol. 53, No. 10, pp. 867.
As advancements have been made in the high integration of semiconductor devices, the more reduction in size is demanded in fabrication, the more severe the requirements become. Accordingly, there have been requested improvements in both directionality in RIE and the alleviation of damage to the devices due to etching. In particular, in the case of RIE, since the radiation of highly energized ions towards a substrate determines the accuracy of the directionality, the more the energy of ions increases, the more the collisions of the ions against the substrate develop, thereby impairing the substrate.
A decrease in the energy of ions may alleviate the aforementioned damage due to etching, but it will result in less anisotropic etching, in other words, less directional etching. Therefore, it can be said that the energy of ions in RIE is specified by the balance between the damage to the substrate and the accuracy of anisotropy in etching, hindering a further improvement in the accuracy of etching. This is referred to in a journal entitled "Semicon NEWS" October, 1981, pp. 31.
Accordingly, attention is now directed towards an optical chemical reaction process which utilizes optical excitation without the use of charged particles (ions). Specifically, in the optical chemical reaction process, reactive gas molecules are transformed into free radicals by means of optical energy, and the reaction of the free radicals with the substrate creates reactive products. Etching is accomplished by knocking out substrate atoms with the free radicals. By virtue of such etching utilizing the optical chemical reaction, damage to the substrate due to etching can be reduced to a minimum.
However, in etching employing a conventional optical chemical reaction, gases are activated by the optical chemical reaction and etching is effected by a simple reaction, i.e., the contact of the activated gases with the substrate. If the activated gases attack optically non-irradiated regions, the regions will be etched, thereby producing an undercut of the substrate. Compared with RIE, etching employing the optical chemical reaction is inferior in its slower reaction rate and etch rate.
Furthermore, in etching using the conventional optical chemical reaction, a low-pressure mercury lamp is used as a light source, and the light wavelength there of is in the range longer than about 100 nm. The use of such light causes diffraction during the radiation, impeding the accurate transfer of a mask pattern onto the substrate. Etching utilizing the optical chemical reaction is described in a journal entitled "SURFACE CHEMICAL" in 1984, vol. 5, No. 4, pp. 435.